1. Field of the Invention
This invention relates to the recovery of non-ferrous metals, such as aluminum, from dross.
2. Brief Description of the Prior Art
Dross is a material which forms on the surface of molten non-ferrous metal, such as aluminum or zinc, during remelting, metal holding and handling operations when the molten metal is in contact with a reactive atmosphere. Dross normally consists of metal oxides entraining a considerable quantity of molten free (unreacted) metal, and for economic reasons it is desirable to extract the free metal before discarding the residue. Recovery can be carried out by treating the dross in a furnace at a high temperature. For this purpose, several furnaces have been devised and are presently being used; such furnaces are normally heated with an external heat source, such as fuel- or gas-operated burners, plasma torches, or electric arcs.
In aluminum processing-operations, for example, the dross, which normally contains at least 50% Al metal, is skimmed-off from the surface of the molten metal in a smelting or similar furnace and is usually loaded into special containers or pans where it is cooled and then it is stored, before being processed in a dross treating furnace which, as mentioned above, is heated with an external heat source.
The use of fuel- or gas-operated burners for heating the dross in a dross treating furnace, in order to recover the aluminum contained therein, has the major drawback of requiring the addition of salt fluxes such as NaCl or KCl, used to increase the percentage of aluminum recovery. Apart from the fact that such salt fluxes increase the cost of the operation, they also lead to increased pollution and are, therefore, environmentally undesirable.
The use of a plasma torch as, for instance, disclosed in U.S. Pat. No. 4,952,237 of Aug. 28, 1990, or of an electric arc as disclosed in U.S. Pat. No. 5,245,627 of Sep. 14, 1993 permits the above mentioned drawback to be overcome. Indeed, the use of plasma or arcs creates higher temperatures in the furnace and thus avoids the necessity of adding salt fluxes. However, both technologies use electricity which in many cases may be more expensive than using fuel or gas heating. Furthermore, the use of plasma or arc requires a significant capital investment in power supplies, controller and other related equipment.
As mentioned in U.S. Pat. No. 4,952,237, it has also been proposed to extract the liquid metal from dross by mechanical compression of the hot dross removed directly from a furnace. Such process requires expensive equipment and high dross temperatures and is limited by these factors to relatively large scale operations. Moreover, such approach does not directly address the disposal problems because the residues will still contain a large quantity of free metal.
It has also been proposed in the case of aluminum dross to induce and maintain burning or thermitting of the dross under controlled conditions by working the dross in an inclined rotary barrel open to the atmosphere or subjected to oxygen injection as disclosed, for example, in U.S. Pat. No. 5,447,548, of Sep. 5, 1995. This permits a certain portion of the metal content to be consumed in order to recover the remainder. This method has the drawback of resulting in poorer metal recovery as some of the metal is burned to provide the heat required for the process.
In U.S. Pat. No. 5,308,375 of May 3, 1994, the furnace heating by a plasma torch is followed by oxygen injection prior to metal tapping. This results in a direct heating of the charge during the separation process which, according to this patent, results in a significant reduction of the plasma power time and of the total cycle time. However, such procedure will undoubtably result in combustion of some of the recoverable metal separated from the dross.
In Canadian Patent Application No. 2,116,249, a gas or fuel burner is used to heat the charge. When the charge reaches a certain temperature, an oxidizing agent such as oxygen is injected onto the charge in the belief that only the unrecoverable finest aluminum particles would be combusted in providing heat for the process. This opinion is shared by U.S. Pat. No. 5,308,375 mentioned above. In both of these processes, oxygen is injected prior to metal tapping in the belief that the recoverable metal would not react with the oxygen and therefore the metal recovery rate would not be affected. No data is presented to support this contention. However, comparative tests conducted at the Hydro-Quebec Research Laboratory on several hundred tonnes of aluminum drosses have shown that dross treatment in an inert atmosphere such as argon produced a metal recovery rate higher by as much as 7% than the treatment conducted in open air; this data (published in "Proceedings of the International Symposium on Environmental Technologies: Plasma Systems and Applications", Volume II, Oct. 8-11, 1995, Atlanta, Ga., U.S.A., p.546) indicates that the recovery rate is likely to be affected by injection of an oxidizing agent onto the charge itself, before tapping the metal.